The present invention relates generally to amplifying apparatus, and, more particularly, to amplifying apparatus for efficiently amplifying both an analog, frequency modulated signal, and a composite modulated signal comprising both an amplitude modulated portion and a phase modulated portion.
Many communication systems transmit information between a transmitter and a receiver by modulating an information signal upon an electromagnetic wave.
Various techniques have been developed to modulate the information signal upon the electromagnetic wave to form thereby the modulated electromagnetic information signal. For instance, amplitude modulation,frequency modulation, and phase modulation are three modulation techniques frequently utilized to modulate an information signal upon the electromagnetic wave.
In general, an amplitude modulated signal is formed by modulating the information signal upon the electromagnetic wave such that the information signal modifies the amplitude of the electromagnetic wave corresponding to the value of the information signal. The frequency of the electromagnetic wave does not vary, and the information content of the modulated signal is contained in the shape, or amplitude, of the signal. A frequency modulated signal, conversely, is formed by altering the frequency of the electromagnetic wave corresponding to the value of the information signal. The amplitude of the electromagnetic wave does not vary, and the information content of the modulated signal is contained in the variation of the frequency of the signal. A phase modulated signal is formed by altering the phase, but not the frequency, of the electromagnetic wave corresponding to the value of the information signal. Similar to the amplitude of the frequency modulated signal, the amplitude of the electromagnetic wave does not vary. The information content of the phase modulated signal is contained in the variation of the phase of the signal. Because the amplitudes of frequency modulated signals and phase modulated signals do not vary, these modulated signals are referred to as constant envelope signals.
Receivers constructed to receive the modulated signals contain circuitry to detect, or to recreate, the information signal modulated upon the electromagnetic wave. This detection or recreation process is referred to as demodulation. Receivers are constructed to permit demodulation of signals modulated by the various modulation techniques.
The electromagnetic wave upon which the information signal is modulated is referred to as the carrier wave, and is characterized by the frequency thereof; such frequency is referred to as the carrier frequency. The information signal, once modulated upon the electromagnetic wave, is positioned, in frequency, close to, or centered about, the carrier frequency characterizing the electromagnetic wave. Modulated, electromagnetic information signals of different carrier frequencies may be transmitted simultaneously as long as the information signals modulated thereupon do not overlap in frequency.
Governmental, both national and international, regulatory authorities regulate the number of, location of, and transmission strength of signals transmitted at frequencies in various frequency bands of the frequency spectrum. Regulation of transmission is necessary to ensure that overlapping of simultaneously transmitted signals does not occur.
A one hundred megahertz band of the electromagnetic frequency spectrum (extending between 800 megahertz and 900 megahertz) is allocated in the United States for radiotelephone communication, such as, for example by the radiotelephones utilized in a cellular, communication system, wherein a modulated, electromagnetic information signal is transmitted between the radiotelephone and a base station.
A cellular, communication system is created by positioning numerous base stations at spaced-apart locations throughout a geographical area. Base stations receive and transmit information signals. Each base station receives the electromagnetic signals transmitted by the cellular, radiotelephone when the phone is located in the vicinity thereof. Each portion of the geographical area containing a base station is defined to be a cell. The electromagnetic information signal transmitted by the cellular, radiotelephone, when positioned in a particular cell, is received by the base station which defines that cell.
The cellular, radiotelephone modulates an information signal upon an electromagnetic wave to produce thereby the modulated electromagnetic information signal which is transmitted to at least one of the base stations positioned throughout the geographical area. The carrier frequency characterizing the electromagnetic wave upon which the information signal is modulated is of a value within the band allocated for radiotelephone transmission. The base station receives the transmitted electromagnetic information signal, and demodulates the signal. The base station, in turn, is connected, typically by a wire or a cable, to a conventional telephone system. The information signal, once demodulated from the electromagnetic information signal, is supplied by the base station to the telephone system to be transmitted therealong.
Transmission circuitry with each of the base stations modulates information signals upon carrier waves by a modulation technique, such as one described hereinabove, to permit transmission to the radiotelephones. Communication to and from the radiotelephones is thereby effectuated.
Simultaneous transmission of modulated electromagnetic information signals to and from numerous cellular, radiotelephones is possible by transmitting the electromagnetic information signals at various different frequencies throughout the allocated frequency band.
Increased usage of cellular, radiotelephones has resulted in a commensurate increase in the utilization of the frequency band allocated for radiotelephone communication. As a result, the allocated frequency band of many cells of various geographical areas has become saturated, as oftentimes, electromagnetic information signals are transmitted simultaneously throughout the entire frequency band. Other frequency bands allocated for other uses are similarly frequently saturated.
Various attempts have been made to increase the information-transmission capacity of cellular, radiotelephone communication systems. For example, increases in the number of base stations in a geographical area commensurately increases the number of cells defined thereby. Increasing the number of cells in a geographical area permits an increase in the number of signals which may be simultaneously transmitted within the geographical area. However, increases in the number of base stations positioned throughout a geographical area are limited by the interference between radio telephones. When base stations are positioned too close to one another, signals transmitted by a single radiotelephone are received by multiple base stations, interfering with radiotelephones in the other cells trying to use the same channel. Additionally, increasing the number of base stations positioned throughout a geographical area increases the cost of the cellular, communication system.
Existing cellular communication systems utilize a frequency modulation technique and the modulated, electromagnetic information signal formed thereby is continuously transmitted. Because the modulated, electromagnetic information signal is continuously generated, only one signal may be transmitted at a specific frequency at any particular time. When more than one signal is transmitted simultaneously at the same frequency, the signals interfere with one another.
Discrete modulation techniques have been developed which permit multiple transmissions at a single frequency. Suggestions have been made, therefore, to convert the information signal, prior to modulation thereof upon the electromagnetic wave, into a discrete, encoded signal. Two or more signals may be simultaneously transmitted by transmitting discrete portions of each of the signals in sequence in discrete bursts.
Discrete, encoded signals are modulated upon an electromagnetic wave by a composite modulation technique. The composite, modulation technique is a combination of amplitude and phase modulation wherein the information content of the composite modulated information signal formed thereby is contained in the amplitude and/or the phase variations of the signal.
Receivers, sometimes referred to as decoders, can be constructed with circuitry to permit reconstruction of a particular one, or each, of the several discrete, encoded signals transmitted at a particular frequency.
Because two or more information signals may be transmitted at the same frequency, the information-transmission capacity of the frequency band allocated for radiotelephone communication may be increased two-fold or greater.
However, because existing analog base stations defining the cells of a cellular communication system must be replaced with base stations capable of decoding discretely encoded information signals, conversion of the cellular, communication systems will occur gradually. As the cellular system base station infrastructure is gradually converted from analog receivers to discrete decoders, some of the channels of a cell of a cellular, communication system will comprise base station receivers having decoders for decoding the discretely encoded signals; other channels of a cell will comprise base station receivers having only existing, conventional analog demodulators. During this conversion period, a radiotelephone must be capable of transmitting both a frequency modulated information signal, and a composite modulated information signal. Therefore, the radiotelephone must contain circuitry to frequency modulate with a continuous wave information signal and to composite modulate with a discrete, encoded signal (the radiotelephone must additionally contain circuitry to encode an information signal into the discrete codes to form the discrete, encoded signal).
Once the information signal, or discrete, encoded signal is modulated upon an electromagnetic wave, the modulated information signal must be amplified to a power level suitable for transmission over extended distances. Conventionally, an analog information signal is modulated upon an electromagnetic wave by the above-described frequency modulation technique. Discrete, encoded information signals, however, are modulated upon an electromagnetic wave by a composite modulation technique which, as stated hereinabove, results in both amplitude modulation and phase modulation. An amplifier which amplifies a composite modulated signal (here, having the discretely encoded information signal modulated thereupon) must be operated in the linear mode in order to preserve the shape of the wave and the information contained therein. Conversely, an amplifier which amplifies a frequency modulated signal should be operated in the saturation mode in order to most efficiently amplify the signal.
A dual-mode radiotelephone permitting transmission of both conventional, frequency modulated signals, and composite modulated signals may be constructed having first circuitry for frequency modulation, and second circuitry for composite modulation of the discrete, encoded signals. A first amplifier may be included with the first circuitry, and constructed to be in a saturation mode, in order to most efficiently amplify the frequency modulated signal. A second amplifier may be included with the second circuitry, and constructed to be in a linear mode, in order to amplify accurately the composite modulated signal.
However, the dimensions of the radiotelephone must be minimized in order to minimize the radiotelephone size. Minimization of the number of circuit parts permits minimization of the radiotelephone size.
Therefore, a radiotelephone operable in both an analog (frequency modulation) and a discrete mode (composite modulation) having a single amplifier for amplifying both a frequency modulated signal and an amplitude modulated signal would be advantageous. However, because the amplifier must be operated in the linear mode to amplify a composite modulated signal, and in a saturation mode to amplify efficiently a frequency modulated signal, existing radiotelephone constructions having a single linear amplifier would operate to transmit an analog, frequency modulated electromagnetic signal very inefficiently.
What is needed, therefore, is amplifying apparatus which may be alternately operated in the linear mode and the saturation mode to efficiently transmit either a composite modulated signal or a frequency modulated signal.